In the biomedical field, devices and elements such as those listed above as examples need to have very different characteristics, firstly from the mechanical standpoint in view of the objective of acting on a part of the human body (such as a tooth, for example, as part of an orthodontic application) and secondly from the biological standpoint, to avoid or minimize the reactions or consequences relating to contact by the device with the part of the human body or organ.
Some of the technical characteristics that are sought or even necessary include the largest possible recoverable elasticity range (property of superelasticity), low rigidity, excellent chemical biocompatibility, high resistance to corrosion and sterilization products, ease of machining and cold work, and increased hardness and resistance to surface wear.
In a known manner, attempts have been made to reconcile these constraints, which are generally contradictory with each other. The superelastic and/or shape memory alloys that are currently in use in the biomedical area are of the titanium-nickel type.
However, it is known that nickel is allergenic for the body and can lead to inflammatory reactions, in spite of the usefulness of its mechanical properties, particularly those of superelasticity and/or shape memory. Besides, Ti—Ni alloys offer mediocre machinability, leading to the premature breaking of endodontic files (see for example: Oiknine M., Benizri J., REV. ODONT. STOMATO. 36 (2007) 109-123) and are sometimes difficult to form when cold.
These known alloys have a superelastic property because of stress-induced destabilization of the (cubic) parent beta phase by transformation into a reversible (orthorhombic) alpha″ martensitic phase (Kim H. Y., Ikehara Y., et al, ACTA MATERIALIA 54 (2006) 2419-2429).
Further, nickel-free titanium alloys (called ‘Gum Metals’, Saito T., Furuta T. et al, SCIENCE 300 (2003) 464-467) are known and are considered to be superelastic, because even if they do not show martensitic transformation under stress, they have low rigidity and very high recoverable elasticity.
Besides, nickel-free titanium alloys are known in the French patent 2 848 810, the U.S. patent application 2007/0137742 and the patent application WO 2005/093109.
Nevertheless, the alloys proposed in that prior art do not satisfactorily meet all the required criteria overall, both in terms of their mechanical properties and those of biocompatibility, particularly at the surface.
For example, in respect of biological compatibility, the French patent above offers a surface treatment of the alloy by depositing nitride, using a plasma based technique.
However, this known technique is not satisfactory. Plasma depositing does not make it possible to deposit an even coat of nitride. That has harmful or adverse consequences in the case of devices or elements with particular shapes or parts or areas that are not easily accessible (such as concavities or the like).
Further, the French patent describes a method that does not apply to a shape memory and/or superelastic alloy.
While most alloys are therefore made from titanium and nickel, superelastic alloys made from nickel-free titanium have been proposed recently, which are particularly easily deformable when cold. The article in the JOURNAL OF THE MECHANICAL BEHAVIOR OF BIOMEDICAL MATERIALS 3 (2010) 559-564, by Bertrand E., Gloriant T. et al “Synthesis and characterisation of a new superelastic Ti-25Ta-25Nb biomedical alloy” shows such nickel-free titanium alloys.
Thus, the method according to the invention makes it possible to solve the problems of the prior art by proposing the manufacture of a titanium alloy for biomedical applications with superelastic and/or shape memory properties and surface treatment, which meets all the mechanical conditions stated above and which is further an improvement on the prior art as regards surface hardness, ease of cold working and machining, and also resistance to sterilization, while being also perfectly biocompatible.